British Pat. No. 973,377 discloses a family of ester imide resins made by reacting together at least one polybasic acid or a functional derivative thereof, and at least one polyhydric alcohol or functional derivative thereof, at least one of the reactants having at least one five-membered imide ring between the functional groups of the molecule. It further discloses that the reactants can be heated in a commercial cresol mixture, then further diluted in a mixture of naphtha and cresol and used as an enamel for coating copper wire to produce a hard, thermally resistant insulation therefor. U.S. Pat. No. 3,426,098 describes ester imide resins in which all or part of the polyhydric alcohol comprises tris(2-hydroxyethyl)isocyanurate.
Okada, Japanese Pat. No. 51-16342 (published Feb. 9, 1976) prepares wire enamels by reacting polyester ingredients to a low molecular weight, then dissolving them in a monoether of a polyalkylene glycol. Ito, Japanese patent publication No. 49-59478 (published Dec. 3, 1975) makes wire enamels by heating polyester ingredients to a molecular weight just before ultimate solubility is lost, then dissolves them in a glycol ether solvent.
Keating, U.S. Pat. Nos. 4,119,605, 4,119,608; and 4,119,758, incorporated herein by reference, discloses low molecular weight polyester-imide resins which are soluble in nonphenolic solvents by incorporating a monoether of a diethylene glycol or a triethylene glycol into the resin. This differs from the Japanese disclosure because the glycol ether is used as a reactant. Keating states that the use of the monoether of a glycol as a reactant serves to control the molecular weight and solubility of the polymer and permits the manufacture of wire enamels which contain relatively non-toxic solvents in place of phenols and also permits the attainment of enamels having economical high solids contents. All of these wire enamels are prepared under conditions in which the formation of the polyester or polyester imide is substantially complete, as shown by the cessation of distillation of reactive by-products. Keating states that the acid number of his products is usually below 6. In all of the working examples, the low acid numbers and distillation under vacuum as a last step before adding solvent indicate that there are no terminal acid groups in such polyesters or polyester imides. The foregoing patents and publications are incorporated herein by reference.
It has now ben discovered that if acid-terminated polyester imide resins are prepared by heating in the presence of a monoether or monoester of a glycol or a polyglycol and the heating is stopped before all volatiles have distilled off and the acid number is still 6 or above, the acid-terminated polyester imide can then be dissolved in a solvent comprising predominantly the same, or different, polar organic monoethers or monoesters of alkylene glycols or polyalkylene glycols to produce superior wire enamels in terms of flexibility retention on aging. If the reaction is taken to low acid numbers--as required in the prior art, flexibility retention suffers, as will be shown hereafter.